This invention relates to articles of footwear and is particularly concerned with improvements in the soles of articles of footwear. The term sole is used herein to denote a part of an article of footwear which in use extends beneath at least the instep of the foot of the user and beneath the fore-part of the foot. As will become apparent from what follows, some of the soles to which the improvements are applied also extend rearwards so that in use they are situated beneath the user's heel.
That part of an article of footwear which in use extends beneath the instep of the foot is often referred to as the waist or shank. As the waist or shank is usually narrower than the heel and the fore-part, it tends to be the least stiff part of the sole so that in use, when the user is walking, it tends to flex transversely more readily than the fore-part or the heel. This tends to be uncomfortable for the user, and to overcome or reduce that difficulty it has been common practice for many years to incorporate in some types of footwear a shank-stiffener in the waist or shank, a typical form of shank-stiffener comprising a flexible and resilient strip of metal extending lengthwise of the waist or shank.
While the use of a shank-stiffener is often satisfactory in walking shoes, which generally have rigid heels and have soles of which the fore-parts made of materials such as leather that have a significant degree of inherent stiffness, the use of a shank-stiffener in that way may well be less satisfactory in articles of footwear of other kinds, and in particular in articles of footwear having soles that are relatively thin and are made of materials, such as some plastics materials, that have little inherent stiffness and resilience. Articles of footwear used in certain sports and games have soles of that kind. The provision of a shank-stiffener in a sole of that kind could well lead to problems for although it would reduce the tendency of the relatively narrow waist or shank to flex too readily, it would lead to a tendency for the sole to bend in a zone immediately forward of the shank-stiffener and would thus be likely to be uncomfortable in use.
The present invention was primarily developed to provide a sole so designed and constructed as to overcome or reduce problems of that kind, but as will become apparent from what follows, the present invention is of more general application than that. As used herein, the term "sole" shall refer to an "outsole", i.e. that portion of the sole below the shoe upper and to which the upper is directly secured, unless the context clearly indicates otherwise.
From one aspect the present invention consists in a sole (as hereinbefore defined) for an article of footwear characterised in that it incorporates stiffening means comprising a first portion operative to stiffen the waist or shank of the sole and a second portion operative both to stiffen the fore-part of the sole, though to a lesser extent than the first portion stiffens the waist or shank, and to provide resilience to cause or assist in the return of the fore-part of the sole to or towards an unflexed state after being subjected, in use, to transverse flexure as the result of flexure of the user's foot at the ball of the foot.
From a second aspect the present invention consists in an article of footwear incorporating a sole which is in accordance with the first aspect of the present invention.
A sole embodying the present invention differs from those previously provided in that it can have, throughout substantially its entire length, physical properties which are selected so as to be both appropriate to the use to which the sole is to be put and comfortable for the user. In particular, the first portion of the stiffening means reduces or overcomes the inherent tendency in most articles of footwear for the waist or shank to flex transversely, when in use, with less resistance than may be suitable or desirable, while the second portion of the stiffening means causes or assists in causing the fore-part of the sole to return to or towards its normal or unflexed state after it has been flexed transversely in the neighbourhood of the ball of the user's foot.
Preferably the arrangement is such as to avoid the presence of a zone of relatively low resistance to flexure adjacent to and in front of the first portion of the stiffening means. To this end the forward end of the first portion may be integral with or united to the rearward end of the second portion. Alternatively the forward end of the first portion may lie ahead of the rearward end of the second portion so that the first and second portions overlap each other lengthwise of the sole.
The stiffening means preferably comprises one or more preformed components incorporated in a sole body. If there are two or more components they may be made from the same material or from different materials. The component or at least one of the components may be formed as a moulding of a plastics material or the like; an acetal resin may be appropriate or a material loaded with fibres such as glass or carbon fibres. Alternatively the component or at least one of the components may be formed by vacuum-forming.
The first portion is preferably of progressively reduced cross-section towards its forward and so that its stiffness is progressively reduced. The first portion may be relatively thick to ensure the necessary stiffness in which case it may be solid or of longitudinally grooved or corrugated form so as to reduce the quantity of material used in its construction.
The second portion may be thinner than the first portion and may comprise two or more elements which are laterally spaced apart and extend generally lengthwise of the fore-part of the sole. The elements are preferably joined together at or near their forward ends.
The sole body is preferably moulded around the stiffening means and may be formed from a plastics material or a similar material which is wear-resistant and flexible. Polyurethane is a suitable material. The arrangement may be such that the stiffening means and the sole body adhere to each other or become bonded to each other but that is not essential and the arrangement may be such that the stiffening means and the sole body are so shaped that they interengage one another and so remain in engagement without reliance on their being adhesively interconnected or being bonded together. Interengagement may be effected by providing holes or other openings in the stiffening means which are entered by the sole body as the latter is formed so as to resist relative movement between the sole body and the stiffening means.
The first portion of the stiffening means is preferably visible from the underside of the sole so as to provide a visible indication of its presence to the user or purchaser. Similarly, at least a part, that preferably being a forward part, of the second portion of the stiffening means is preferably visible from the underside of the sole. Either or both of those portions may be visible through an aperture in the sole body or may be visible through a transparent or translucent window in the sole body. Alternatively, or in addition, the sole body or at least part thereof may be made of transparent or translucent material so that it reveals the presence of the stiffening means to a user or purchaser.